Humidity sensing is of great importance in many industrial applications, including monitoring of pharmaceutical, biomedical, food/beverage storage and logistics, electronics, automotive and semiconductors industries. For the food monitoring application for example, measuring the relative humidity inside the package is necessary to monitor the quality of the product, as excessive moisture content may lead to premature spoilage. In such application, the miniaturization of the sensors is also a key element.
The vast majority of the humidity sensors nowadays commercially available are capacitive sensors. Such sensors use a material, whose dielectric constant change as a function of relative humidity, and the corresponding change of the capacitance is recorded. However, the dependence of the capacitance on relative humidity is in most cases not linear and substantial hysteresis is usually encountered at high humidity values, due to a slower diffusion time in the moisture sensitive film during the dehumidifying process.
An alternative to a capacitive sensor is a resistive sensor. The basic configuration is the same as that of a capacitive humidity sensor, but the dielectrics layers are replaced by conducting layers whose conductivity depends on the absolute or relative humidity values. In this case, humidity measurements hysteresis still remains a problem, especially if the sensors are not used continually. Long-term stability of the sensor can only be guaranteed by continual operation and attentive maintenance. In, e.g., industrial applications, shelf life before usage of the sensor is an unknown factor.
Humidity sensors are known from, e.g., following patent publications, all incorporated herein by reference.
CN1455250 discloses a sensor that comprises a substrate, an oxide layer and a capacitance electrode. The oxide layer is on the substrate and the capacitance electrode with the lead wire is on the oxide layer. The capacitance electrode is made from the material suitable to the technique of preparing standard CMOS. The passivation layer is setup on the capacitance electrode. The comb type parallel structure of the aluminum anode increases the sensing value of the capacitance. The substrate connected to the ground reliably eliminates interference outside so as to raise the accuracy. The deposited passivation layer prevents the electrode from corrosion of water vapor absorbed by polyimide. The heating circuit prepared by using the comb type parallel polysilicon makes desorption time shorter. This configuration provides the advantages of high sensitivity, linearity, good hysteresis and reliability for a long time.
JP2002062252 addresses as the problem to be solved: to quickly and inexpensively diagnose in real-time a life and a degree of deterioration of equipment having a metallic portion to provide a recipe for maintenance of the equipment. This document proposes a system that has an environment sensor group for measuring environmental factors in the environment where the equipment having the metallic portion is installed, a diagnostic client for monitoring outputs of sensors, a diagnostic server for estimating the life and the degree of the deterioration based on the sensor outputs transmitted from the diagnostic client via the Internet to provide a diagnostic result therein and the recipe to the diagnostic client via the Internet, and a diagnostic database accumulated with a data group required for diagnosis.
US published patent application 20060213264 teaches the following.
Conventionally, for preventing electric devices from dewing and from short circuits resulting from water cooling systems therein, the electric devices include dewing sensors to detect the vapor saturation that cause a feedback control signal to be sent for driving excitators. The characteristic curves of the conventional dewing sensors, however, are too smooth to detect the dewing reaction when the humidity is between 89-90% relative humidity. Furthermore, because of the high cost of the conventional dewing sensors made of metallic oxides by integrated circuit process, the conventional dewing sensors are not extensively used in some low-cost electrical devices. Another dewing sensor is proposed. The dewing sensor includes a substrate having at least two electrodes, at least two comb electrodes, and a detecting layer. The comb electrodes contact the two electrodes on the substrate, and the detecting layer includes a derivative of cellulose formed on the comb electrodes. For detecting the humidity, the two electrodes of the dewing sensor are fastened by a clamping apparatus and connected to a multimeter. The temperature and humidity are determined by a standard temperate-humidity controller. After stabilizing the dewing sensor, the impedance between the electrodes of the dewing sensor is output.
U.S. Pat. No. 4,965,698 discloses a capacitance humidity sensor with a dielectric film core which is in contact with a pair of polymeric conductive layers bonded to opposite faces of the core. The dielectric core is made of a polymeric material having a dielectric constant which varies substantially linearly with humidity, such as a polyimide or polyparabanic acid. The conductive layers are made of a polymeric material having conductive particles, such as carbon particles, dispersed therein. Such conductive layers provide superior performance and corrosion resistance in comparison to the metal films commonly employed at that time.